Robert Reisz can almost see the muscles twitching in the ancient dinosaur embryos.

The University of Toronto paleontologist points to the tiny thigh bones, which he can hold between his fingers. The bones show signs of being tugged and shaped as the creatures squirmed and stretched inside their eggs – eggs that were swept out of their nests and into what Reisz and his colleagues are calling the “world’s oldest dinosaur embryo bonebed.”

The jumble of tiny bones and bits of eggshell, which they have unearthed on a hillside in China, is seen as a major find that provides a glimpse of the early development of gigantic creatures that roamed the planet 190 million years ago.

“We are opening a new window into the lives of dinosaurs,” Reisz said in a release issued with a report on the bonebed published Wednesday in the journal Nature.

He says the find will have a major impact on understanding of the creatures, enabling scientists to track the growth of embryonic dinosaurs.

Reisz and his colleagues have so far extracted and studied more than 200 bones from the bonebed.

The researchers say the bones are from 20 embryos that probably came from several nests, as the bones are at different stages of development. Much of their work and anaylsis has focused on the largest bones, the femurs or thighbones.

Reisz and his colleagues report that they can see evidence of how muscles contracted and pulled on the femurs as they grew, helping shape the bones. This suggests that dinosaurs, like modern birds, moved around inside their eggs, says Reisz: “It represents the first evidence of such movement in a dinosaur.”

The embryos are from a group of long-necked dinosaurs, sauropodomorph Lufengosaurus, that roamed Earth in the early Jurassic.

Adults grew up to nine metres in length, but the bonebed indicates they started out in eggs about three times as wide as a chicken egg.

“Based on the size of the bones we can predict that the eggs were about nine cm in diameter,” says Reisz. “A hatchling may have been about 25 cm in total length.”

His coauthor Timothy Huang, at the National Chung Hsing University in Taiwan, discovered the signs of the embryos “washing down a small hillock” in southwest China about three years ago.

“We dug in and found the bone bed,” says Reisz.

So far the team has excavated only a square metre of the bonebed, known as Lufeng, and found not only bones but fragments of delicate eggshells, less than 100 microns thick. They’ve also found organic material, which may be collagen fibres, inside the embryonic bones.

“A find such as the Lufeng bonebed is extraordinarily rare in the fossil record, and is valuable for both its great age and the opportunity it offers to study dinosaur embryology,” says Reisz.

Margaret's work covering science - and science controversies - has taken her to the Arctic to see the effects of global warming, to Cape Canaveral for space launches and into Ottawa's paper labyrinth ... read moreto reveal how the Canadian government has been muzzling scientists.View author's profile